1. Field of the Invention
The present invention relates to a slot type shadow mask for use in a shadow mask type color picture tube.
2. Description of the Background Art
As shown in FIG. 7, a shadow mask type color picture tube typically comprises three electron guns 1, a fluorescent screen 2 and a shadow mask 3. Electron beams B from the guns 1 cause the screen 2 to glow red, green and blue. The shadow mask 3 permits only certain of the electron beams B to pass through beam apertures 3A such that stripes of fluorescent material on the fluorescent screen 2 are caused to glow in response to image signals.
The mask 3 includes about a hundred thousand to hundreds of thousands of the strip-shaped beam apertures 3A. The apertures 3A have a horizontal dimension of 0.08 to 0.25 mm and a vertical dimension of 0.3 to 1.0 mm. The apertures 3A are defined within a steel plate which is 0.1 to 0.3 mm thick. The apertures 3A are arranged in a regular bricklaying manner. The apertures 3A are opened/formed by etching.
To form bright color images, it is necessary to increase the proportion of electron beams which are transmitted through the shadow mask 3. The width of each aperture is a function of (1) the arrangement of the different fluorescent materials on the screen 2 and (2) the transverse pitch between the beam apertures 3A. The bridges cannot be too thin and the length of each beam aperture cannot be too long or else the shadow mask will be broken during the molding, working and mounting of the shadow mask on the color picture tube.
The slot type shadow mask 13 shown in FIG. 8 has been proposed in U.S. Pat. No. 4,293,792. The mask 13 provides increased electron beam transmission.
The mask 13 has a large number of longitudinal beam apertures 13A which are vertically and transversely arrayed on a single mask plate member 14. Bridges 16a and 16b are alternately interposed between the beam apertures 13A. The bridges 16a are displaced toward a front surface S.sub.1 and the bridges 16b are displaced toward a rear surface S.sub.2. The mask plate member 14 has a thickness T. The thickness of the bridges 16a, 16b is less than T.
The shadow mask 13 is fabricated by applying photoresist films to front and rear surfaces of a thin metal plate, drying the same, contact-printing shadow mask patterns on the front and rear surfaces, spray etching and then separating the photoresist films.
To displace the bridges 16a toward the
surfaces S.sub.1 and to displace the bridges 16b toward the rear surfaces S.sub.2, it is necessary to etch the single mask plate member 14 from the front and rear surfaces. To do this, the etching speed, the pressure of the etching solution and the like must be strictly controlled. This means that the dimensions of the beam apertures 13A as well as the dimensions of the bridges 16a and 16b are simultaneously controlled during etching on the premise that the thickness of the thin metal plate, the thickness of the photoresist films, the sensitivity of the photoresist material and the like are not variable. However, it is necessary to perform etching while most emphasizing the opening dimensions of the beam apertures 13A in practice. Hence, if the thickness of the materials is variable, the thicknesses of the bridges 16a and 16b are correspondingly variable If the bridges are too thin, strength is reduced and the bridges may be broken during molding/working. In sum, it is difficult to fabricate the mask 13 by etching.
Further, the positions of the beam apertures and the stripes of fluorescent materials must correctly coincide with each other.
However, since the shadow mask forms an anode of the color picture tube with the fluorescent materials, the percentage of electron beam transmission through the shadow mask is only about 20%. Power loss at the anode reaches 80%. When anode power is about 25 watts (W) in a 20-inch color picture tube, 20 W of power is expended by the shadow mask. This causes the temperature of the shadow mask to increase by about 40.degree. C. The mask expands by about 100 .mu.m as a result. Such thermal expansion prevents coincidence of the electron beams and the fluorescent materials. The picture tube's color purity is therefore reduced.
In this regard, generally known is a shadow mask which is formed by two plate members so that corresponding slot holes of the second plate member are brought into close contact with slot holes of the first plate member or opposed to the same with small clearances thereby to increase the strength of the shadow mask, as disclosed in Japanese Patent Laid-Open No. 79170/1974. Japanese Patent Laid-Open No. 131676/1974 discloses preventing local thermal expansion by stacking two sliced shadow masks (plate members) so that a large number of openings entirely overlap each other. However, the percentage of electron beam transmission in these masks is changed by displacement of the positions of the slot holes (openings) of the overlapped plate members due to thermal expansion of master patterns (which are employed for printing shadow mask patterns), distortion caused during etching of the shadow mask, inaccurate alignment of the plate members and the like.
Further, transmission irregularity caused by inaccurate hole configurations within a plate member is compounded when such plate members are overlapped.
U.S. Pat. No. 4,374,452 discloses a post-focusing type color picture tube with means for improving the brightness of displayed images by increasing the percentage of electron beam transmission in a color selector.